Various printing techniques use a roll, usually named “anilox roll” or “metering roll”, for the purpose of supplying a calibrated amount of ink to another roll or to the substrate to be printed. The main applications of the anilox roll are described below:                In the flexography and offset printing techniques the anilox roll brings the calibrated amount of ink to another roll, named “plate roll”, which contains the image to be printed, and which in turn will print the image to the substrate, which can be for example a ribbon of paper, plastic foil, or carton board.        In the gravure printing technique the anilox roll brings the calibrated amount of ink directly to the substrate, which can be for example a ribbon of paper, plastic foil, or carton board. Similarly, in other industries the anilox roll is used for bringing a calibrated amount of e.g. glue, varnish, to a substrate.        All these applications have in common the requirement for the anilox roll to deliver an amount of liquid, like ink, varnish, glue, that must be precisely calibrated, otherwise the print quality is adversely affected.        
An anilox roll is composed of a textured surface, made of cells or grooves, which are filled with a liquid, e.g. ink, during the printing process. This arrangement of cells, named “engraving”, is nowadays engraved with a tightly focused laser beam in a ceramic coating, usually made of thermally sprayed chrome oxide. The ceramic coating usually lies on an intermediate coating, usually made of thermally sprayed nickel-chrome, which serves both as an anticorrosion barrier and a bond between the ceramic coating and the base material. The base material is usually made either of steel or aluminum.
The quantity of liquid that the engraving of an anilox roll can carry, named “volumetric capacity”, represents one of the key factors determining the quantity of liquid that is transferred to the plate roll, and finally to the substrate. The other important factors affecting the transfer rate of the anilox roll are among others the liquid properties especially the viscosity and the surface tension, the pressure of the anilox roll to the plate roll or to the substrate, the machine speed, or in other terms the rotation rate of the anilox roll. An incorrect volumetric capacity of the anilox roll may come from dirtiness lying in the bottom of the cells, e.g. dried ink, wear of the ceramic coating, or incorrect manufacturing of the anilox roll. The volumetric capacity is measured as the quantity of liquid that a unit surface of the roll can carry, the usual units being cm3/m2 and BCM/inch2, in which BCM meaning billion cubic micrometer.
The usual method to measure the volumetric capacity of an anilox roll follows the principle described by the patent U.S. Pat. No. 4,628,728A: a small amount of colored ink, e.g. 10 microliters, is applied with a precision micropipette on the roll surface, the said ink is spread with a doctor blade which is a steel blade of typically 0.15 mm thickness, so that the cells of the engraving are precisely filled with the ink to their top, the surface of the ink blot on the roll is imaged on a foil of paper or plastic, the area of the said surface is measured with a planimeter or a scanner. This method requires a precisely calibrated micropipette and planimeter, and trained operator in order to apply and spread the ink in a way that is repeatable. Moreover the liquid rheology, e.g. surface tension and viscosity, must be adapted to the size of the cells and surface tension of the ceramic in order to correctly fill the cells. This method is not well suited to a printing shop due to the fact the components are fragile and the operator must be specially trained for that method of measurement. The precision of this method typically lies between +/−5% and +/−10%.
A variant of the method described in U.S. Pat. No. 4,628,728A is the object of U.S. Pat. No. 5,235,851: the measurement liquid is forced through a channel of a transparent strip, which is applied on the roll surface to be measured. The length in the channel reached by the liquid is inversely proportional to the volumetric capacity. This method is easier and quicker to implement compared with the former described method, however, it is less precise.
A third method to measure the volumetric capacity of anilox roll proceeds with a light interferometer, e.g. patent DE-101,54,348-A1: light is focused through a microscope objective onto the roll surface, the light reflected from the walls of the cells is reflected back to the microscope and compared with the light sent by the microscope. The distance of the portion of the wall onto which the light was focused is deduced. By scanning the focused light down the cell to its bottom a three-dimensional image of the cells is obtained, from which the volumetric capacity can be computed. This method has the drawbacks of being expensive and not fully reliable: the surface of the chrome oxide ceramic that is usually used in anilox rolls is dark, and hence a small portion of light is reflected back, which negatively influences the instrument precision.
A fourth method, disclosed in patent U.S. Pat. No. 5,438,864, applies a fluorescent fluid on the roll surface to be measured, ultraviolet light is directed towards the said fluid, and the fluorescent light emitted by the said fluid is measured by the instrument. The volumetric capacity of the roll is linked to the amount of fluorescent light detected.
This method has the advantage of using a quantity of measurement fluid that does not need to be calibrated, on the contrary of the two first methods previously described, but the instrument precision is poor due to the fact that the amount of fluorescent light emitted is dependent on the shape of the cells being pyramid-shaped or bowl-shaped, as well as the color of the ceramic, e.g. light grey or dark grey.
A fifth method is described in patent JP-7,148,914-A: a contact finger is sensing the depth of the point of contact, the cell profile is then deduced from a scan of the contact finger along the surface. This method is not applicable for most anilox rolls used nowadays, as the cell diameter is too small, 10 to 50 microns, for the contact finger to reach the cell walls.
Some processes get around the measurement of the volumetric capacity by modifying this volumetric capacity during the printing process, in order to reach the desired quantity of liquid transferred from the roll: e.g. U.S. Pat. No. 5,370,052 and U.S. Pat. No. 6,308,623-B1. Neither of these methods has proved precise nor reliable.
Considering the above, there is the need for a method which is precise, reliable, and easy to use in an industrial environment. The implementation of such a method would allow the printer to monitor the efficiency of his anilox rolls, and hence gain control of one key parameter of the printing process.
Other methods for measuring the volumetric capacity of rotogravure chrome-plated rolls are known. These rolls are characterized by an outer layer made of chrome-hardened copper, with the plurality of cells of the engraving being in the said copper, whereas for the ceramic anilox rolls that are the subject of this invention the plurality of cells are engraved in a non-conductive ceramic layer. One of said methods is disclosed by the French patent FR-1,540,970 and the American patent U.S. Pat. No. 3,931,570. It uses an low-frequency alternating magnetic field which magnetizes the ferromagnetic base material of the roll, and measures the resulting net magnetic field. This method is applied over a non-engraved area on the roll surface, and over an engraved area on the roll surface. A differential signal is then deduced, which is related to the volumetric capacity of the engraving.
Another one, described by the American patent application US-2004/118,180-A1 uses a high-frequency alternating field which generates eddy currents in the non-ferromagnetic base material, and measures the resulting inductance.
The PCT application WO-2010/055,320-A1 measures the capacity of the roll outer layer by means of two electrodes applied at a distance on the roll surface, with an alternative electric field between them.
These four patents (FR-1,540,970, U.S. Pat. No. 3,931,570, US-2004/118,180-A1, WO-2010/055,320-A1) have in common that they measure metallic-coated rolls that are used in the rotogravure printing process: these rolls are characterized by an outer layer made of chrome-hardened copper, with the plurality of cells of the engraving being in the said copper. The said outer layer is thus conductive, which is essential for the methods described in these patents to work: they use the property that the plurality of cells of the engraving, which are formed in a metallic layer, alters the electro-magnetic properties of the said metallic layer.
The methods described in the said four patents are therefore not applicable to the anilox rolls that are mainly used in the flexographic printing process, due to the fact that the outer layer of these rolls is made of chrome oxide ceramic, the said outer layer being highly electrically non-conductive and non-ferromagnetic. Therefore, no electro-magnetic signal can be generated in the coating that contains the engraving, and thus the methods described in the said four patents are not dependent on the volumetric capacity of the anilox roll that has to be measured, which makes them not applicable to ceramic-coated anilox rolls.
The present invention avoids the necessity to apply a precisely calibrated amount of liquid in the roll engraving, as needed in the five patents U.S. Pat. No. 4,628,728A, U.S. Pat. No. 5,235,851, DE-101,54,348-A1, U.S. Pat. No. 5,438,864, and JP-7,148,914-A. The present invention uses any quantity of liquid, or in other words a quantity of liquid that is not calibrated. This represents a significant improvement, considering the difficulty to precisely sample a quantity of liquid in the microliter range as needed in the five said patents. The present invention uses a liquid that contains nanometer-scale ferromagnetic particles, whereas the said five patents use non-ferromagnetic liquids.
The present invention uses an alternating magnetic field in order to measure the surface density of said ferromagnetic particles, from which the volumetric capacity of the engraving is deduced. It is thus a concentration of ferromagnetic particles that is measured. The four said patents, in contrast, measure the ferromagnetic properties of a metallic outer coating, and do not make use of a liquid with ferromagnetic particles.
The particular parameter setting of the oscillating magnetic field for the measurement of the ferromagnetic particle surface density has to be specifically matched with the characteristics of anilox rolls in order to yield a precise and reproducible measurement, as will be described in the next chapters. Users of anilox rolls normally do not use apparatus working with an oscillating magnetic field, nor are they using liquids containing ferromagnetic particles, which underline the novelty of the present invention. The combination of a textured ceramic coating with a liquid containing ferromagnetic particles, probed by a magnetic field, has to our knowledge never been proposed.